lib/56323: fvwrite assertion failures during install

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         56323
>Category:       lib
>Synopsis:       fvwrite assertion failures during install
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    lib-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Jul 22 11:30:00 +0000 2021
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, soruce date >= 2021.07.16.12.34.10
>Organization:
>Environment:
System: NetBSD
Architecture: i386
Machine: i386
>Description:

When installing NetBSD-current/i386 using sysinst, multiple error
messages like this one are printed to the terminal during the set
extraction phase:

  assertion "fp->_w >= 0" failed: file "/tmp/build/2021.07.22.04.46.29-i386-laptop/src/lib/libc/stdio/fvwrite.c", line 60, function "flush_adj"

The installation succeeds nonetheless.

The problem started around the time of this commit:

  2021.07.16.12.34.10 christos src/lib/libc/stdio/fvwrite.c 1.28

>How-To-Repeat:

>Fix: